The proteasome, (also refered to as multicatalytic protease (MCP), multicatalytic proteinase, multicatalytic proteinase complex, multicatalytic endopeptidase complex, 20S, 26S, or ingensin) is a large, multiprotein complex present in both the cytoplasm and the nucleus of all eukaryotic cells. It is a highly conserved cellular structure that is responsible for the ATP-dependent proteolysis of most cellular proteins (Tanaka, Biochem Biophy. Res. Commun., 1998, 247, 537). The 26S proteasome consists of a 20S core catalytic complex that is capped at each end by a 19S regulatory subunit. The archaebacterial 20S proteasome contains fourteen copies of two distinct types of subunits, α and β, which form a cylindrical structure consisting of four stacked rings. The top and bottom rings contain seven α-subunits each, while the inner rings contain seven β-subunits. The more complex eukaryotic 20S proteasome is composed of about 15 distinct 20–30 kDa subunits and is characterized by three major activities with respect to peptide substrates. For example, the proteasome displays tryptic-, chymotryptic-, and peptidylglutamyl peptide-hydrolytic activities (Rivett, Biochem. J., 1993, 291, 1 and Orlowski, Biochemisty, 1990, 29, 10289). Further, the proteasome has a unique active site mechanism which is believed to utilize a threonine residue as the catalytic nucleophile (Seemuller, et al., Science, 1995, 268, 579).
The 26S proteasome is able to degrade proteins that have been marked by the addition of ubiquitin molecules. Typically, ubiquitin is attached to the ε-amino groups of lysines in a multistep process utilizing ATP and E1 (ubiquitin activating) and E2 (ubiquitin-conjugating) enzymes. Multi-ubiquitinated substrate proteins are recognized by the 26S proteasome and are degraded. The multi-ubiquitin chains are generally released from the complex and ubiquitin is recycled (Goldberg, et al., Nature, 1992, 357, 375).
Numerous regulatory proteins are substrates for ubiquitin dependent proteolysis. Many of these proteins function as regulators of physiological as well as pathophysiological cellular processes. Alterations in proteasome activity have been implicated in a number of pathologies including neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, as well as occlusion/ischaemia reperfusion injuries, and aging of the central nervous system.
The ubiquitin-proteasome pathway also plays a role in neoplastic growth. The regulated degradation of proteins such as cyclins, CDK2 inhibitors, and tumor suppressors is believed to be important in cell cycle progression and mitosis. A known substrate of the proteasome is the tumor suppressor p53 which is involved in several cellular processes (see, e.g., Ko, L. J. Genes Dev., 1996, 10, 1054). Tumor suppressor p53 has been shown to induce apoptosis in several haematopoietic cell lines (Oren, M., Semin. Cancer Biol., 1994, 5, 221). Induction of p53 leads to cell growth arrest in the G1 phase of the cell cycle as well as cell death by apoptosis. Tumor suppressor p53 degradation is known to be carried out via the ubiquitin-proteasome pathway, and disrupting p53 degradation by inhibition of the proteasome is a possible mode of inducing apoptosis.
The proteasome is also required for activation of the transcription factor NF-κB by degradation of its inhibitory protein, IκB (Palombella, et al., Cell, 1994, 78, 773). NF-κB has a role in maintaining cell viability through the transcription of inhibitors of apoptosis. Blockade of NF-κB activity has been demonstrated to make cells more susceptible to apoptosis.
Several inhibitors of the proteolytic activity of the proteasome have been reported. See, for example, Kisselev, et al., Chemistry & Biology, 2001, 8, 739. Lactacystin is a Streptomyces metabolite that specifically inhibits the proteolytic activity of the proteasome complex (Fenteany, et al., Science, 1995, 268, 726). This molecule is capable of inhibiting the proliferation of several cell types (Fenteany, et al., Proc. Natl. Acad. Sci. USA, 1994, 91, 3358). It has been shown that lactacystin binds irreversibly, through its β-lactone moiety, to a threonine residue located at the amino terminus of the β-subunit of the proteasome.
Peptide aldehydes have been reported to inhibit the chymotrypsin-like activity associated with the proteasome (Vinitsky, et al., Biochemistry, 1992, 31, 9421; Tsubuki, et al., Biochem. Biophys. Res. Commun., 1993, 196, 1195; and Rock, et al., Cell, 1994, 78, 761). Dipeptidyl aldehyde inhibitors that have IC50 values in the 10–100 nM range in vitro (Iqbal, M., et al., J. Med.Chem. 1995, 38, 2276) have also been reported. A series of similarly potent in vitro inhibitors from α.-ketocarbonyl and boronic ester derived dipeptides has also been reported (Iqbal, et al., Bioorg. Med. Chem. Lett. 1996, 6, 287, U.S. Pat. Nos. 5,614,649; 5,830,870; 5,990,083; 6,096,778; 6,310,057; U.S. Pat. App. Pub. No. 2001/0012854, and WO 99/30707).
N-terminal peptidyl boronic ester and acid compounds have been reported previously (U.S. Pat. Nos. 4,499,082 and 4,537,773; WO 91/13904; Kettner, et al., J. Biol. Chem., 1984, 259, 15106). These compounds are reported to be inhibitors of certain proteolytic enzymes. N-terminal tri-peptide boronic ester and acid compounds have been shown to inhibit the growth of cancer cells (U.S. Pat. No. 5,106,948). A broad class of N-terminal tri-peptide boronic ester and acid compounds and analogs thereof has been shown to inhibit renin (U.S. Pat. No. 5,169,841).
Various inhibitors of the peptidase activities of the proteasome have also been reported. See, e.g., Dick, et al., Biochemistry, 1991, 30, 2725; Goldberg, et al., Nature, 1992, 357, 375; Goldberg, Eur. J. Biochem., 1992, 203, 9; Orlowski, Biochemistry, 1990, 29, 10289; Rivett, et al., Archs. Biochem. Biophys., 1989, 218, 1; Rivett, et al., J. Biol. Chem., 1989, 264, 12215; Tanaka, et al., New Biol., 1992, 4, 1; Murakami, et al., Proc. Natl. Acad Sci. USA, 1986, 83, 7588; Li et al., Biochemistry, 1991, 30, 9709; Goldberg, Eur. J. Biochem., 1992, 203, 9; and Aoyagi, et al., Proteases and Biological Control, Cold Spring Harbor Laboratory Press (1975), pp. 429–454.
Stein et al., U.S. patent application Ser. No. 08/212,909, filed Mar. 15, 1994, report peptide aldehydes useful for reducing in an animal both the rate of loss of muscle mass and the rate of intracellular protein breakdown. The compounds are also said to reduce the rate of degradation of p53 protein in an animal. Palombella, et al., WO 95/25533, report the use of peptide aldehydes to reduce the cellular content and activity of NF-κB in an animal by contacting cells of the animal with a peptide aldehyde inhibitor of proteasome function or ubiquitin conjugation. Goldberg and Rock, WO 94/17816, report the use of proteasome inhibitors to inhibit MHC-I antigen presentation. Stein, et al., U.S. Pat. No. 5,693,617 report peptidyl aldehyde compounds as proteasome inhibitors useful for reducing the rate of degradation of protein in an animal. Inhibition of the 26S and 20S proteasome by indanone derivatives and a method for inhibiting cell proliferation using indanone derivatives are reported by Lum et al., U.S. Pat. No. 5,834,487. Alpha-ketoamide compounds useful for treating disorders mediated by 20S proteasome in mammals are reported in Wang et al., U.S. Pat. No. 6,075,150. France, et al., WO 00/64863, report the use of 2,4-diamino-3-hydroxycarboxylic acid derivatives as proteasome inhibitors. Carboxylic acid derivatives as proteasome inhibitors are reported by Yamaguchi et al., EP 1166781. Ditzel, et al., EP 0 995 757 report bivalent inhibitors of the proteasome. 2-Aminobenzylstatine derivatives that inhibit non-covalently the chymotrypsin-like activity of the 20S proteasome have been reported by Garcia-Echeverria, et al., Bioorg. Med. Chem. Lett., 2001, 11, 1317.
Some further proteasome inhibitors can contain boron moieties. For example, Drexler et al., WO 00/64467, report a method of selectively inducing apoptosis in activated endothelial cells or leukemic cells having a high expression level of c-myc by using tetrapeptidic boronate containing proteasome inhibitors. Furet et al., WO 02/096933 report 2-[[N-(2-amino-3-(heteroaryl or aryl)propionyl)aminoacyl]amino]-alkylboronic acids and esters for the therapeutic treatment of proliferative diseases in warm-blooded animals. U.S. Pat. Nos. 6,083,903; 6,297,217; 5,780454; 6,066,730; 6,297,217; 6,548,668; U.S. Patent Application Pub. No. 2002/0173488; and WO 96/13266 report boronic ester and acid compounds and a method for reducing the rate of degradation of proteins. A method for inhibiting viral replication using certain boronic acids and esters is also reported in U.S. Pat. No. 6,465,433 and WO 01/02424. Pharmaceutically acceptable compositions of boronic acids and novel boronic acid anhydrides and boronate ester compounds are reported by Plamondon, et al., U.S. Patent Application Pub. No. 2002/0188100. A series of di- and tripeptidyl boronic acids are shown to be inhibitors of 20S and 26S proteasome in Gardner, et al., Biochem. J., 2000, 346, 447.
Other boron-containing peptidyl and related compounds are reported in U.S. Pat. Nos. 5,250,720; 5,242,904; 5,187,157; 5,159,060; 5,106,948; 4,963,655; 4,499,082; and WO 89/09225, WO/98/17679, WO 98/22496, WO 00/66557, WO 02/059130, WO 03/15706, WO 96/12499, WO 95/20603, WO 95/09838, WO 94/25051, WO 94/25049, WO 94/04653, WO 02/08187, EP 632026, and EP 354522.
A great interest exists, as evidenced by the above references, in drugs which can modulate proteasome activity. For example, molecules capable of inhibiting proteasome activity can arrest or delay cancer progression by interfering with the ordered degradation of cell cycle proteins or tumor suppressors. Accordingly, there is an ongoing need for new and/or improved inhibitors of proteasome.